1. Technical Field of the Invention
The present invention relates generally to a light valve apparatus and a display system using same, and more particularly to a liquid-crystal light valve and a projection display system for projecting bright fine images.
2. Background Art
Large-screen projection systems are known in the art wherein optical images are formed on light valves according to image signals and light beams are directed onto the optical images for projection onto a large projection screen through a projection lens. In recent years, there has been proposed a projection display system using liquid-crystal light valves. For example, Japanese Patent First Publication No. 62-133424 to T. Ono et al. discloses a projection display system which utilizes three active matrix liquid crystal panels for three primary colors: red, green, and blue for projecting high-quality images. Each of the active matrix liquid crystal panels is formed with a twisted nematic (TN) liquid crystal and carries thin-film transistors (TFTs) as a switching element for each pixel. In addition, Japanese Patent First Publication No. 2-250015 to T. Tanaka et al. teaches a compact projection display system using TFT liquid crystal panels as disclosed in T. Ono et al. and a translucent projection screen to arrange all optical systems and electric circuits within a small-sized cabinet.
In general, a TFT liquid crystal panel may malfunction when intense light enters a TFT. For avoiding this drawback, a metal thin film black matrix is usually provided on a liquid crystal layer on a light-receiving side glass substrate for isolating the TFT and circuit wiring from the intense light. The TFT liquid crystal panel uses only light passing through the black matrix, and the brightness of projected images is determined in proportion to an aperture ratio of the liquid crystal panel (i.e., an area ratio of all apertures of the black matrix to the entire image-displaying surface). Thus, utilizing light reaching non-aperture portions of the black matrix will increase the brightness of the projected images, resulting in energy efficiency being improved. Accordingly, there has been a prior art approach wherein a lens array plate is arranged adjacent a light-receiving side of a liquid crystal panel for brightening projected images. For example, Japanese Patent First Publication No. 1-189685 to S. Matsuo et al. discloses an arrangement wherein an array of positive lens elements whose each optical axis lies in alignment with an opening of each pixel of a liquid crystal panel, is arranged at a light-receiving side of the liquid crystal panel. In this arrangement, substantially parallel light beams incident to the liquid crystal panel are converted by the positive lens elements into convergent beams which, in turn, pass through the openings of black matrixes with high efficiency as an optical beam diameter becomes smaller at a location of the black matrix. Thus, an effective aperture ratio of the liquid crystal panel is increased as compared with an aperture ratio of the black matrix. In addition, Japanese Patent First Publication No. 2-262185 to H. Hamada proposes an arrangement wherein a lens array plate which includes a transparent plate carrying lens arrays on its both side surfaces for decreasing the cross-sectional diameter of a light beam projected from the light-projecting lens array smaller than the diameter of the lens element of the light-receiving lens array, is arranged at a light-receiving side of a liquid crystal panel for increasing the brightness of projected images.
Projecting fine images using a projection display system including a combination of a TFT liquid crystal panel and a lens array plate requires increasing the number of pixels per unit area of the liquid crystal panel, resulting in pixel pitches being decreased. When light-source real images reduced in size which are formed on the pixels of the liquid crystal panel through the lens array plate are greater in dimension than the pixels, the effective aperture ratio in the case of parallel incident light beams is improved, however, projected images do not become bright. Reducing the size of the light source real images requires shortening the focal length of each of positive lens elements of the lens array plate. This requires use of a thin light-receiving side glass substrate of the liquid crystal panel. However, the thin light-receiving side glass substrate makes it difficult to provide uniform thickness of a liquid crystal layer. For solving this problem, Japanese Patent First Publication No. 2-302726 to Y. Ito et al. teaches the arrangement of lens elements in a light-receiving side glass substrate. The fabrication of a distributed index type lens array using an ion exchange process commonly requires a glass substrate containing alkali ions. This gives rise to a problem related to the deterioration of characteristics of TFTs under the influence of melted alkali ions. In addition, formation of the lens array between two glass substrates requires a combination of the glass substrates having different refractive indexes. Since such glass substrates generally show different coefficients of thermal expansion, it is difficult to uniform the thickness of the liquid crystal layer over a wide temperature range. In either case, as long as positive lens elements are formed inside a light-receiving side glass substrate, it is difficult to display high-quality images on the liquid crystal panel. The prior art approaches have suffered from the drawback in that it is difficult to project bright fine images.
Usually, video cameras require a compact and light-weight body for easy carrying. For reducing the overall size of the camera, the use of a liquid crystal panel in a view finder has been proposed. In addition, display of high-quality images on the liquid crystal panel has been sought. This requires reduction in size of a display screen of the liquid crystal panel with need for a great number of pixels. In other words, it is necessary to decrease pixel pitches in the liquid crystal panel. This arrangement, however, reduces an aperture ratio of the liquid crystal panel, resulting in the brightness of displayed images being reduced. For brightening the displayed images, it is necessary to provide a bright light source, but resulting in consumption of electric power being increased. This also leads to reduction in continuous operating lifetime produced by one-time battery charging.